build: bump prerelease version number

pyproject.toml

 [tool.poetry]
 name = "pycgtool"
-version = "2.0.0-alpha.5"
+version = "2.0.0-alpha.6"
 description = "Generate coarse-grained molecular dynamics models from atomistic trajectories."
 authors = ["James Graham <j.graham@soton.ac.uk>"]
 license = "GPL-3.0-only"